1. Technical Field
This application relates to signal processing and, more particularly, to detecting noise from an adjacent channel.
2. Related Art
Communication systems may use frequency modulation (FM) or amplitude modulation (AM) to transmit information to a remote party. A system using frequency modulation conveys information over a carrier wave by varying the frequency of the carrier wave. A system using amplitude modulation conveys information by varying the amplitude of the carrier wave while holding the frequency of the carrier wave constant. In some implementations, frequency modulation may be more robust than amplitude modulation against noise and interference.
The FM broadcast band may fall within the very high frequency range (VHF; in Germany: UltraKurzWelle, UKW) and other frequency ranges. The FM broadcast band may be split into channels by defining a channel spacing that leaves a buffer between channels in attempt to reduce interaction between adjacent channels. In some systems, the FM broadcast band may use a channel spacing of 100 kHz, with a 25 kHz buffer above and below the center frequency of the channel. In some systems, the bandwidth of an FM signal may actually be wider than the provided channel spacing. The bandwidth of an FM signal may be predicted according to: BW≈2*(Δf+faudio), where faudio is the maximum modulating frequency and Δf is the frequency deviation in the channel. An FM signal with a maximum modulating frequency of 15 kHz and a frequency deviation of 75 kHz would have a bandwidth of about 180 kHz. Therefore, a channel spacing of ˜200 kHz or more would result in a system without overlapping frequencies of adjacent channels, but then fewer channels would be available.
Although the bandwidth of an FM signal may be wider than the provided FM channel of 100 kHz, two adjacent channels may exist without much interference as long as a sufficient distance separates the two stations. FM broadcasting has a relatively low distance range. When two channels are distant from each other, the signal energy of one channel may be too low to lead to noise in the other channel. However, an adjacent radio station may still cause interference in another signal, especially when the carrier frequencies are nearby and the signal strength of the adjacent radio channel is large enough compared to the signal strength of the signal of interest. Therefore, a need exists for a system for improved detection of noise from an adjacent channel.